I add "spot blacks" - patches of black that act as shading
- and also some lighter highlights to the outlines on the monsters in
the foreground.

I put grey tones onto the image - both flat greys, and gradients, which
are patches where one colour blends into another. Here I use gradients
that blend from white to grey, to show the fall-off of light coming
through the open door.

I also start to add rendering - little pen strokes to make the scene
look gritty and dirty - I add score marks in the grime in the floor
where the door has opened.

To make a guide for the score marks (inset),
I made a set of concentric circles with the Blend Tool, then distorted
them to fit the perspective grid.